Loudspeaker

ABSTRACT

A loudspeaker comprises a voice coil and a diaphragm attached to the voice coil. The diaphragm has a generally rectangular outer shape, and comprises an outer rim (A 1 ) having an outer edge at which the diaphragm is fixed in position and an inner section (A 0 ) within the outer rim. The inner section (A 0 ) comprises: an outermost area (A 00 ) which is coupled to the voice coil and having the same generally rectangular outer shape; and an inner area (A 01 ) comprising a periodic rib structure (A 01   p ), with the ribs running parallel to the shorter side of the rectangular outer shape, and two lateral transition areas (A 01   c ) between the edges of the periodic rib structure (A 01   p ) and the shorter side edges of outermost area (A 00 ).

This invention relates to loudspeakers.

A loudspeaker comprises a membrane for generating a sound pressure wave,and a voice coil attached to the membrane. A control signal applied tothe voice coil causes it to move as a result of the interaction of theresulting electromagnetic field with the magnetic field of a permanentmagnet. The voice coil is typically arranged around or within astationary permanent magnet.

The membrane typically comprises a dome shaped structure, suspended atits outer periphery and attached to the voice coil either at its centreor near to the outer periphery, radially inside the outer suspensionarea.

The dome area needs to have a specified resonance frequency at or abovethe upper limit of the overall system frequency spectrum in order toavoid disturbing effects to the sound pressure level at the useablefrequency band. The resonance frequency of the dome depends on itsstiffness and mass. The mass needs to be as low as possible and thestiffness needs to be tuned to the required resonance frequency.

There is therefore a trade off between the desire to use extra materialto increase the stiffness, and the desire to keep the mass of the domeas low as possible. In particular, to maintain a high resonancefrequency of the dome, the stiffness needs to be high. However, the massof the dome area needs to be as low as possible for improved overallspeaker performance.

The speaker architecture often limits the space for the dynamicallymoving membrane, so that the dome area needs to be as thin as possiblein order not to reduce the space used by the membrane in use.Furthermore, extra component parts should be avoided because of theadditional processing time and process complexity and additionalmanufacturing costs.

A known approach for achieving the requirements of the dome area is touse a stiff but light plate in addition to the flexible membranestructure. This can cause some difficulties in the production andincreases the cost.

An alternative is to shape the dome using the membrane material. In thisway, the mass can be kept as low as possible. Often, this results in aspherical shape for the dome, but this has the problem that thestructure is too high.

There is therefore a need for a membrane design which increases thestiffness of the dome with minimum use of material and which enables athin design to be formed.

According to the invention, there is provided a loudspeaker comprising:

-   -   a voice coil; and    -   a diaphragm attached to the voice coil,    -   wherein the diaphragm has a generally rectangular outer shape,        and comprises an outer rim having an outer edge at which the        diaphragm is fixed in position and an inner section within the        outer rim, wherein the inner section comprises:    -   an outermost area which is coupled to the voice coil and having        the same generally rectangular outer shape;    -   an inner area comprising a periodic rib structure, with the ribs        running parallel to the shorter side of the rectangular outer        shape, and two lateral transition areas between the edges of the        periodic rib structure and the shorter side edges of outermost        area.

The invention provides a loudspeaker design in which the diaphragm canbe formed from a single component, and the design enables a combinationof large stiffness, small overall thickness and low mass.

The combination of the outermost area and the transition areas providesa closed and stiff frame around the periodic rib area. This frameenables the periodic rib area to be small and thereby increase theresonance frequency by decoupling the forces resulting at the edges ofthe periodic rib area at resonance to the rest of the membrane system.

The periodic rib area has ribs extending between the longer edges. Ateach point across the periodic structure, the longer edges are connectedwith a straight line along a rib. The straight line connection has thefunction of avoiding the periodic rib area being stretched/compressedalong the short dimension axis. The periodic structure in general causesthe periodic rib area to be stiffened in the direction of the smallerrectangle edge.

The lateral transition areas are preferably mirror symmetric about aline parallel to the shorter edges of the rectangular outer shape. Thelateral transition areas can each comprise a dome. Each such dome cancomprise a first portion which provides a height increase from a lowestheight at the boundary with the periodic rib structure to a maximumheight, and a second portion which provides a more sharply curved heightdecrease from the maximum height to the lowest height where thediaphragm is attached to the voice coil. The first portion has a concaveshape, which provides a stiffening effect.

The second portion has a smooth transition to the first portion and adiscontinuous transition to the lowest height where the diaphragm isattached to the voice coil.

The period rib structure preferably extends fully to the outermost areaat the longer side edges of the outermost area. Thus, the ribs extendfully from top to bottom (with the rectangle arranged with the long sideleft to right), and the ribs thereby define a rigid frame with theoutermost area.

The height of the periodic rib structure preferably does not exceed theheight of the lateral transition areas.

The longer side of the generally rectangular diaphragm outer shape ispreferably less than 100 times the diaphragm thickness. The generallyrectangular outer shape preferably comprises a rectangle with roundedcorners. In this case, the sixe of the transition areas along the longerrectangle side direction can correspond to the size of the of the cornercurves of an inner edge of the outermost area. This means the ribsoccupy the maximum rectangular area of the diaphragm.

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1A shows a cross section of the membrane of a loudspeaker of theinvention;

FIG. 1B shows the membrane of FIG. 1A in plan view;

FIG. 2 shows the membrane of FIG. 1A in perspective view;

FIG. 3 shows lateral transition areas of the design of FIG. 1A and 1Bmore clearly;

FIG. 4 shows the rectangular shape of the periodic rib area;

FIG. 5 shows the curved edges of the first portion of the lateraltransition areas of FIG. 2;

FIGS. 6 and 7 are used to show more clearly some of the areas definedfor the purposed of explaining the invention; and

FIG. 8 shows a mobile phone including the loudspeaker of the invention.

The invention provides a loudspeaker with a particular design ofmembrane. The invention aims to provide a desired stiffness of the innerpart of the membrane without needing extra parts or materials andwithout violating the restriction of a limited dome height.

The stiffness of the dome is proportional to the frequency responsespectrum of the loudspeaker at a certain frequency band (between 6 kHzand 12 kHz) depending on the resonance frequency of the dome itself andthe resonance frequency of the coil which is connected to the dome.

The advantage of the invention is a large cost reduction duringmanufacturing of the transducer by not using extra parts and bondingprocesses to have a defined stiffness of the dome.

FIG. 1A shows a cross section of the membrane of the invention, and FIG.1B shows the membrane in plan view. The cross section of FIG. 1A isalong the x-axis shown in FIG. 1B.

The diaphragm has a generally rectangular outer shape as shown in FIG.1B. By “generally” rectangular, is meant the shape is rectangular,though with rounded corners. Also, the sides do not of course need to beperfectly straight.

The diaphragm can be formed from polymer plastics: (PAR, PEN, PET, PEEK,PC, PA, . . . ). It may be a single material or a compound structure.Silicones, rubbers, paper and fiber compounds are also possible.

The diaphragm has an outer rim (A1) having an outer edge at which thediaphragm is fixed in position. The outer edge of the outer rim (A1) ismounted to the loudspeaker enclosure. The outer rim (A1) is the elasticpart of the diaphragm suspension.

The inner edge of the outer rim (A1) defines the start of an innersection (A0) within the outer rim. The inner section (A0) is intended tobe an inelastic part.

The outermost area (A00) of the inner section (A0) is coupled to thevoice coil (C) and has the same generally rectangular outer shape.

Movement of the voice coil thus causes the outer rim (A1) to flex. Theouter rim (A1) allow this movement and therefore is not designed to bevery rigid. It provides a restoring force to a neutral position. Theouter rim (A1) comprises a single dome shape as shown extending betweenthe outermost area (A00) of the inner section (A0) and the fixationpoint of the diaphragm.

The inner section (A0) has an inner area (A01) which includes a centralarea (A01 p) in the form of a periodic rib structure, with the ribsrunning parallel to the shorter side of the rectangular outer shape.

This rib section (A01 p) provides the membrane rigidity and is the partwhich generates the sound pressure wave in response to voice coilmovement. The ribs can have a generally sinusoidal profile.

The inner area (A01) of the inner section (A0) also has two lateraltransition areas (A01 c) between the edges of the central rib area (A01p) and the shorter side edges of outermost area (A00). These transitionareas are only along the shorter sides.

There is a more direct coupling at the end of the ribs to the longersides of the outermost area (A00). This means the structure is veryrigid in the y axis direction.

To give the diaphragm the desired compliance, the structure is lessrigid in the x axis direction, by virtue of the transition areas (A01c).

The transition areas themselves (A01 c) are stiff in both directionsbecause they link to the corner radius. Thus, the width of thetransition regions (i.e. the dimension along the longer x-axisdirection) corresponds to the corner radius of the inner edge of theoutermost area (A00).

The lateral transition areas (A01 c) are mirror symmetric about the yaxis, and they each comprise a dome.

FIG. 2 shows the membrane in perspective view. This shows more clearlyhow the ribs extend almost fully from the top to the bottom of therectangle. The ribs flatten at their ends (flattening to their peaksrather than their troughs), so that they make a smooth transition to theoutermost area (A00). FIG. 2 also shows more clearly that the flatmounting for the voice coil comprises a well which extends all aroundthe generally rectangular shape.

The lateral transition areas are shown more clearly in FIG. 3.

Each dome comprises a first portion (A01 c) which provides a heightincrease from a lowest height at the boundary with the central periodicrib area (A01 p) to a maximum height (H), and a second portion whichprovides a more sharply curved height decrease from the maximum height(H) to the lowest height where the diaphragm is attached to the voicecoil (C). This second portion is the outermost area (A00). The secondportion (A00) has the same shape in a y-axis cross section, so that thecurvature from the highest point down to the well for the voice coil isthe same all around the structure.

Each first portion (A01 c) is in the form of a concave surface, and isgenerally spherical in the regions of the corners.

FIG. 4 shows the rectangular shape of the periodic rib area (A01 p).This rectangle is bounded by longer sides E1 and E2 at the inner edge ofthe outermost area (A00) and by shorter sides Ep1 and Ep2. These shortersides extend between the ends of the corner curves of the inner edge ofthe outermost area (A00), so that the sides E1, E2, Ep1 and Ep2 definethe largest rectangle that can fit within the outermost area (A00).

FIG. 5 shows the curved edges Ec1, Ec2 which define the end of the firstportion (A01 c). The edges Ec1 and Ec2 are formed from the shorter sidesof the edges of the outermost area (A00) but also the curved corners.Because the lateral transition areas (A01 c) are bounded by these curvedcorners, they are also more rigid in the x axis direction than theperiodic rib area (A01 p).

Referring back to FIG. 3, the second portion of the dome (correspondingto outermost area A00) has a smooth transition to the first portion (A01c) and a discontinuous transition to the lowest height where thediaphragm is attached to the voice coil. The smooth transition is at themaximum height H, whereas the transition between the second portion ofthe dome and the periodic rib area (A01 p) is at the lowest height. Thisgives good overall stiffness.

The lateral transition areas (A01 c) have a height (H) which is notexceeded by the height of the innermost periodic rib area (A01 p).

The longer side of the generally rectangular diaphragm outer shape ispreferably less than 100 times the diaphragm thickness.

FIGS. 6 and 7 are used to show more clearly some of the areas definedabove.

FIG. 6 shows the outer rim (A1) and the inner section (A0).

FIG. 7 shows that the inner section (A0) is formed of the outermost area(A00) and the inner area (A01) (which is itself formed of the periodicrib area (A01 p) and the lateral transition areas (A01 c) as shown inFIG. 1B).

FIG. 8 shows a mobile phone 10 including the loudspeaker system 12 ofthe invention.

The invention is of particular interest for miniature speakers, forexample where the overall thickness of the structure is to be kept assmall as possible.

By way of example, the dimensions of the membrane may be such that thedimensions shown in FIG. 1A and FIG. 3 are in the following ranges:

A01 (size of rigid part of diaphragm):

-   -   Width: 2 to 6 mm    -   Length: 4 to 10 mm    -   Corner Radius: 0.5 to 3 mm

A00 (width of voice coil well and outer curved edge):

-   -   0.3-0.6 mm

A01 c (size of concave part of transition region):

-   -   Width: corresponds to corner radius of A01 (0.5 to 3 mm)    -   Length: corresponds to A01 width (2 to 6 mm)

H (height of structure):

-   -   0.2 to 0.8 mm

Number of periodic ribs:

-   -   4-20 ribs

Length/width ratio of the rectangle:

-   -   1.0-5.0

Thickness of diaphragm:

-   -   10-90 μm

The ribs can be considered to have a transition region towards the area(A00), when they flatten out. The ribs may flatten out over a similardistance to the width of the concave part of the transition region.However, they are fully flattened over a shorter length than the widthof the concave part of the transition region, for example 0.1 to 0.5times that width.

The permanent magnet has not been shown above. The magnet system is anassembly of one, two or three magnets and ferromagnetic pole caps (i.e.soft magnetic iron) to give the magnetic field the right direction.There are magnet parts on the inside of the coil and other parts on theoutside. The speaker needs a so called airgap in the magnet system inwhich the coil can make its vertical displacement. In this way, the coilis essentially surrounded by the magnet system. The design of the magnetsystem is not altered by the use of the diaphragm design of theinvention, and accordingly a detailed description is not provided.Similarly, the speaker circuitry and the mounting of components aretotally routine. The invention resides only in the specific geometricaldesign of the diaphragm.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measured cannot be used to advantage. Any reference signs inthe claims should not be construed as limiting the scope.

1. A loudspeaker comprising: a voice coil; and a diaphragm attached tothe voice coil, wherein the diaphragm has a generally rectangular outershape, and comprises an outer rim having an outer edge at which thediaphragm is fixed in position and an inner section within the outerrim, wherein the inner section comprises: an outermost area which iscoupled to the voice coil and having the same generally rectangularouter shape; an inner area comprising a periodic rib structure, with theribs running parallel to the shorter side of the rectangular outershape, and two lateral transition areas between the edges of theperiodic rib structure and the shorter side edges of outermost area. 2.A loudspeaker as claimed in claim 1, wherein the lateral transitionareas are mirror symmetric about a line parallel to the shorter edges ofthe rectangular outer shape.
 3. A loudspeaker as claimed in claim 1,wherein the lateral transition areas each comprise a dome.
 4. Aloudspeaker as claimed in claim 3, wherein each dome comprises a firstportion which provides a height increase from a lowest height at theboundary with the periodic rib structure to a maximum height, and asecond portion which provides a more sharply curved height decrease fromthe maximum height to the lowest height where the diaphragm is attachedto the voice coil.
 5. A loudspeaker as claimed in claim 4, wherein thesecond portion has a smooth transition to the first portion and adiscontinuous transition to the lowest height where the diaphragm isattached to the voice coil.
 6. A loudspeaker as claimed in claim 1,wherein the period rib structure extends fully to the outermost area atthe longer side edges of the outermost area.
 7. A loudspeaker as claimedin claim 1, wherein the height of the periodic rib structure does notexceed the height of the lateral transition areas.
 8. A loudspeaker asclaimed in claim 1, wherein the longer side of the diaphragm generallyrectangular outer shape is less than 100 times the diaphragm thickness.9. A loudspeaker as claimed in claim 1, wherein the generallyrectangular outer shape comprises a rectangle with rounded corners. 10.A loudspeaker as claimed in claim 9, wherein the side of the transitionareas along the longer rectangle side direction corresponds to theradius of the corner curves of an inner edge of the outermost area. 11.A portable electronic device comprising a loudspeaker, the loudspeakercomprising: a voice coil; and a diaphragm attached to the voice coil,wherein the diaphragm has a generally rectangular outer shape, andcomprises an outer rim having an outer edge at which the diaphragm isfixed in position and an inner section within the outer rim, wherein theinner section comprises: an outermost area which is coupled to the voicecoil and having the same generally rectangular outer shape: an innerarea comprising a periodic rib structure, with the ribs running parallelto the shorter side of the rectangular outer shape, and two lateraltransition areas between the edges of the periodic rib structure and theshorter side edges of outermost area.
 12. A portable electronic deviceas claimed in claim 11, comprising a mobile telephone.